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6 Twice as Sleepy or Half Alert?
of PVT or MSLT findings? Obviously, conceptualising sleepiness in this
manner makes as little sense as would suggesting that hunger can be ‘doubled’ by going without two meals instead of one. Nevertheless, we do use
the language of quantity to describe sleepiness, which is another reason
why language based, subjective scales such as the KSS do have a key role
to play and, in their own way, are just as important and meaningful as
the objective measures.
There are some similar conundrums relating to sleepiness that can be
illustrated by the MSLT. The first is based on the widely acknowledged
average MSLT score being around 12–15 minutes for healthy, good
sleeping adults. It implies that about half the healthy population will fall
asleep faster than this, and suggests that a few minutes or so before falling
asleep they would have been sleepy—sufficiently so to fall asleep. If they
had felt alert prior to entering the MSLT bedroom, then this again shows
that sleepiness can be ‘unmasked’, or maybe even generated in ostensibly
alert people, if they so wish. Of course, it might be argued that this average MSLT latency includes a substantial proportion of the population
with sleep debt, as they fail to attain a potentially desirable much longer
MSLT score, indicative of what might be viewed as ‘full alertness’. But
there is probably little to be gained by asking the participant who has
fallen asleep during the MSLT how sleepy they were prior to dropping
off, as we know that falling asleep itself clouds such a judgement, as will
be seen in the next section.
Another aspect of sleepiness, relating to the normal distribution of
sleep and sleepiness, is as follows. If healthy, fully sleep-satiated people
have their night sleep restricted by a fixed amount, either in absolute or
proportional terms, then due to the normal distribution between people,
some will naturally have shorter MSLT scores than others, despite the
same sleep loss. Alternatively, if the normal sleep lengths for a variety
of healthy sleepers of similar age and sex are ranked so that their MSLT
scores are identical, then presumably there will be a normal distribution
in these sleep durations. A similar finding may well be seen with other
measures of sleepiness despite these constant MSLT scores. These various
normal distributions may well belie more interesting interpretations, not
only in terms of the relationships between sleep durations and MSLT
scores in healthy good sleepers, but have wider implications.
In effect, this all means that for any given ‘quantum of sleepiness’ there
is a normal distribution in the ability of healthy people to fall asleep.
Whilst on the one hand, there will be those people who, naturally, will
need a higher degree of sleepiness before falling asleep, on the other hand,
and excluding those who are actually sleep deprived, there will be some,
in this distribution, who can, if they so wish, fall asleep relatively rapidly but are not necessarily particularly sleepy, as was found in the study
of those patients successfully treated for obstructive sleep apnoea (Sect.
8.2). This ‘high sleepability’, not so much due to sleep loss, is also seen
by us and others [17, 18] in some healthy young adults having very short
MSLT scores, below 6 minutes. To test whether they really were sleepy,
we  gave them a much longer PVT session, lasting half an hour. But
there were no lapses and their reaction times were normal—in fact better
than normal. That is, they seemed quite able to decide whether or not to
fall asleep, when in the appropriate circumstances.
With the possible exception of certain severe clinical conditions (e.g. narcolepsy), sleep does not occur spontaneously from an alert state. There
is always a feeling of sleepiness beforehand, and apart from our findings
clearly demonstrating this [19, 20] other research groups have also done
so . Thus, it is not possible to be alert one minute and asleep the
next—certainly if one has no intention of falling asleep. This has important medico-legal implications for drivers who fall asleep at the wheel
, who often claim that before the collision they had no feeling of
sleepiness and thus fell asleep without forewarning. However, as we have
seen, when given a few minutes to settle down, people have good insight
into any sleepiness.
Somewhat paradoxically, and apart from sleep itself, sleepiness also
clouds one’s memory for recent sleepiness, as we have also found 
when using a real car interactive road driving simulator, where drivers
who had their EEG’s recorded, with their faces monitored for eye closures,
had to report their level of sleepiness (using the KSS) every few minutes
during the drive. Although those having a microsleep at the wheel (seen
in the EEG and by eye closure) are quite able to declare beforehand,
that they feel sleepy, they cannot remember this actual feeling after the
drive, despite remembering saying that they were sleepy. This phenomenon applies to all of us with, for example, our usual failing to remember,
today, the actual feeling of sleepiness and how sleepy we were before bedtime last night and, particularly, when this became noticeable. The same
applies to hunger and thirst—we can seldom remember either in any
detail even a short while after a meal or drink, even though it was clear
at the time that we were hungry or thirsty. The reason is that the human
brain does not have the capacity to remember such pointless information
after the consummatory acts of sleeping, eating and drinking.
Yet, sleepy drivers often reach a point of ‘fighting sleep’ revealed by
their opening the car window (for fresh air), turning up the radio, stretching and so on, which must be self-evident that they are sleepy. Sleep is
a dangerous state, and all living organisms are provided with behaviours
necessary to ensure that they do not fall asleep spontaneously, and have
forewarning to allow them to reach a place of relative safety. But the real
problem, here, is that drivers will often continue to drive in this state,
taking the risk in believing that they are able to remain awake, but are
actually unable to predict if and when they will fall asleep. Of course, if
someone is driving in the small hours of the morning, when they would
usually be asleep in bed, and knowingly have had little sleep that night,
then this is another reason why sleepiness would be self-evident.
In having fallen asleep, and then to know that one has been asleep,
this sleep has to last at least a minute or so. Waking someone up who
has momentarily ‘dropped off’ unexpectedly with an unscheduled sleep
of less than this duration usually results in the sleeper's genuine disbelief
of having been asleep. Sleepiness and the process of falling asleep clouds
one’s ability to know whether one is asleep or awake during this initial
transition period. Which is a reason why most drivers who momentarily
fall asleep for a few seconds at the wheel and have a collision subsequently
have little recollection of actually having fallen asleep.
Sleepiness as a propensity to fall sleep and as an index of the ‘biological need for sleep’ are not necessarily synonymous, as its various measures reflect both essential and less or even non-essential aspects of sleep.
That is, these measurement devices and their varying methodologies also
encompass, to varying extents, different qualitative aspects of sleepiness
apart from its intensity, that depend on, for want of a better term, ‘the
influence of the mind’. Besides, as neither sleep nor sleepiness seem to
reflect uniform, equally proportional or numerically equivalent dimensions, then statistically significant outcomes can sometimes have less biological, psychological or clinical significance, or even obscure real sleep
needs. Furthermore, the more sensitive a test of sleepiness is, especially in
the laboratory environment, then the more it will be able to eke out the
relatively inconsequential aspects of sleep and sleepiness. Thus, levels of
sleepiness ‘unmasked’ in the laboratory by refined tests, but small enough
to be unnoticed by healthy people leading normal lives in more stimulating environments, should make us circumspect in assuming these levels of sleepiness to be realistic, especially when these lead to claims of
chronic ‘sleep debt’ in the general population. It is even possible that
sleepiness can be generated de novo by a particularly dull and tedious
situation. Nevertheless, I have argued that people do have good ‘online’
insight into their own sleepiness, whereas experimental findings indicating otherwise seem largely an artefact of the methodologies. Sleepiness is
potentially life-threatening and for Nature not to have made provision
for us to detect this in ourselves is most unlikely. The real danger for
sleepy people is for them to ‘consciously deny’ their sleepiness and take
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Badly Disrupted Sleep
Inadequate sleep comes in various forms. For example, it can just be too
short, or at the wrong time of the day as in shift work, where both can
produce excessive daytime sleepiness (EDS). Alternatively, this inadequacy can be more of a subjective state, as we have seen with insomnia,
where the patient’s distress is not so evident in objectively defined sleep
when seen with overnight polysomnography (PSG) and absence of EDS.
During normal sleep we naturally move and change position, momentarily, about five times an hour, which usually necessitate short, 2–3 second, waking arousals. Such interruptions are unnoticeable and too short
to cause any real sleep disturbance. However, when they become longer
and more frequent, then this impaired sleep can become an even greater
factor in determining waking well-being than can sleep duration, with
the key sign usually being EDS, depending on the severity of the disruptions. For a sleeper to realise they have woken up during the night and
to have remembered this next morning, they usually have to be actually
awake for at least 30 seconds, more likely a minute or so, which is why
short duration awakenings due to obstructive sleep apnoea (OSA) and
© The Editor(s) (if applicable) and The Author(s) 2016
J. Horne, Sleeplessness, DOI 10.1007/978-3-319-30572-1_9
periodic limb movement disorders of sleep (PLMD, see below) go unnoticed by the sleeper, despite occurring frequently during sleep. Sufferers
are usually oblivious to their sleep having been so grossly disturbed, and
may well believe that they have slept well. Such persistent daytime sleepiness, day after day, month after month, can reach such a chronic level
that the patient ‘forgets’ what it is like to be normally alert, to the extent
that they may well think this sleepiness is ‘normal’ for them. Treatments
(mentioned below) can be remarkably effective and rapid, often dramatically noticeable to the patient, literally overnight, when they can wake up
feeling sufficiently alert to realise how persistently sleepy they had really
Thus, inadequate sleep has marked contrasts, reflected by insomnia on
the one hand, with its hyperarousal and perceived poor sleep, to OSA and
PLMD, with both the latter usually accompanied by EDS and with the
patient often unaware that their sleep is so poor. These latter two are the
most prevalent sleep disorders in terms of severity of sleep disturbances
and provide other important perspectives on sleep quality and sleepiness, which I will describe. The rarer narcolepsy-cataplexy will provide
yet another perspective.
Excessive Daytime Sleepiness (EDS)
The most commonly used subjective scale for assessing EDS, is the
Epworth Sleepiness Scale , which is self-explanatory:
The Epworth Sleepiness Scale (ESS)
How likely are you to doze off or fall asleep in the following situations, in
contrast to just feeling sleepy?
For each of the situations listed below give yourself a score of 0 to 3,
0 = Would never doze
1 = Slight chance
2 = Moderate chance
4 = High chance
Work out your total score by adding up the scores for situations 1–8.
If you have not been in one of these situations recently, think about
how you might have been affected.
Sitting and reading
Sitting inactive in a public place (e.g. cinema, theatre, meeting)
As a passenger in a car for an hour without a break
Lying down to rest in the afternoon
Sitting and talking to someone
Sitting quietly after lunch (when you’ve had no alcohol)
In a car while stopped in traffic
Conventionally, EDS is diagnosed when these scores total 11 and
Obstructive Sleep Apnoea (OSA)
OSA is the commonest form of severe sleep disruption, seen and heard by
very heavy snoring. Patients with OSA can have no EDS, with ESS scores
within the normal range, which suggests that the mechanisms underlying
sleep cope with or adapt to this degree of sleep disturbance and, hence,
symptoms are even more likely to go unnoticed by sufferers. This apparently milder OSA is not accompanied by the hyperarousal as happens
with insomnia. However, when OSA reaches the point to cause EDS,
then it is called ‘OSA syndrome’ (OSAS); that is ‘syndrome’, here, is
synonymous with EDS.
The upper airway at the back of the throat is a rather flabby tube
largely kept open by surrounding muscle tension, normally allowing the
free flow of air in and out of the lungs when breathing during wakefulness. In sleep these muscles relax, and during inhalation this part of the
airway, known as the ‘oropharynx’, sags inwards owing to the lower air
pressure, as happens when breathing in. Too much of this ‘flabbiness’
leads to ‘vibrations’, which is, the more normal, mild snoring. Usually, we
breathe through the nose during sleep, with mouth closed, thus clamping